.ON APRIL 2ND a contraption appearing like a snow device on the deck of the USS Hornet, an invalid aircraft-carrier moored in San Francisco Bay, began creating a moisture of salted aerosol fragments. The scientific partnership behind the venture, the Marine Cloud Whitening (MCB) programme, led due to the College of Washington, is examining whether such sprays could assist to reduce weather adjustment through a form of "geoengineering". What does this term way-- and why is it thus debatable?